1. Field of the Invention
This invention relates to an offset correction circuit for correcting an offset error caused by an input signal of a DC amplifier and a DC amplification circuit including the offset correction circuit.
2. Description of the Related Art
In a conventional DC amplifier for amplifying an input signal and outputting the amplified signal, it is ideal that if an input signal is 0, an output signal becomes 0. However, if just after an input signal of a large amplitude is added, it is set to 0, an output signal does not become 0 and an offset error caused by the input signal occurs. Some offset errors caused by the input signal continue for 10 milliseconds to several ten seconds; the cause of continuation of the offset error for a long time may depend on temperature stability and power supply voltage variation rate.
FIG. 5 is a basic circuit diagram of a DC amplifier. First, occurrence of an offset error depending on the temperature stability resulting from an input signal will be discussed. In FIG. 5, a DC amplifier 5 is made up of a differential amplifier 55 and a voltage amplifier 56. The differential amplifier 55 consists of transistors TR51 and TR52 and resistors R51, R52, and R53. The voltage amplifier 56 consists of a transistor TR53 and resistors R54 and R55. A positive input terminal IN51 is connected to a base of the transistor TR51 and a negative input terminal IN52 is connected to a base of the transistor TR52. An output terminal O51 is connected to the resistor R55. A positive power supply is connected to a power supply input terminal V51 and a negative power supply is connected to a power supply input terminal V52.
A signal is input through the positive input terminal IN51 or the negative input terminal IN52 and is output to the output terminal O51. Normally, a part of signal output from the output terminal O51 is fed back into the negative input terminal IN52, but not fed back in FIG. 5 for simplicity. The transistors TR51 and TR52 are of the same structure and characteristics and the resistors R51 and R52 are of the same resistance value. Further, if the applied voltage to the positive input terminal IN51 is set to 0 and the negative input terminal IN52 is grounded, the current flowing into the resistors R51 and R52 is reduced to half the current flowing into the resistor R53, and the characteristics of the transistor TR53 and the resistor R55 are adjusted so that the voltage of the output terminal O51 becomes 0.
First, when a positive voltage is applied to the positive input terminal IN51, in the differential amplifier 55, a collector current of the transistor TR51 increases and a collector current of the transistor TR52 decreases. As the collector current of the transistor TR51 increases, the voltage of the resistor R51 increases and the collector-emitter voltage of the transistor TR51 decreases. Normally, to widen the differential input range, the collector-emitter voltage of the transistor TR51 or TR52 is large as compared with the voltage of the resistor R51 or R52. Thus, the decrease rate of the collector-emitter voltage of the transistor TR51 is small as compared with the increase rate of the current of the transistor TR51. Therefore, power consumption of the transistor TR51 is increased and the device temperature of the transistor TR51 rises. Since power consumption of the transistor TR52 is decreased and the device temperature of the transistor TR52 lowers, the temperature of the transistor TR51 becomes higher than that of the transistor TR52. The temperature difference between the transistors TR51 and TR52 is as in the following expression (1): ##EQU1##
where
Q(51), Q(52): Heat capacity of transistor TR51, TR52, PA1 P(51), P(52): Heating value of transistor TR51, TR52, PA1 .DELTA.T(51), .DELTA.T(52): Temperature difference between device of transistor TR51, TR52 and environment, and PA1 .theta.(51), .theta.(52): Heat resistance between device of transistor TR51, TR52 and environment.
Next, the applied voltage to the positive input terminal IN51 is set to 0. If there is no temperature difference between the transistors TR51 and TR52, the currents flowing into the resistors R51 and R52 become the same and the voltage of the output terminal O51 becomes 0. However, since the temperature of the transistor TR51 is higher than that of the transistor TR52 as described above, the base-emitter voltage of the transistor TR51 becomes smaller than the base-emitter voltage of the transistor TR52. Thus, the base current of the transistor TR51 becomes larger than the base current of the transistor TR52 and the collector current of the transistor TR51 becomes larger than the collector current of the transistor TR52. As the collector current of the transistor TR51 becomes larger than the collector current of the transistor TR52, the voltage of the resistor R51 becomes larger than the voltage of the resistor R52 and a positive offset error voltage occurs at the output terminal O51.
The offset error voltage occurring at the output terminal O51 is caused by the temperature difference between the transistors TR51 and TR52; the cause of temperature difference occurrence between the transistors TR51 and TR52 is eliminated already by setting the applied voltage of the positive input terminal IN51 to 0, but the temperature difference caused by the power consumption difference between the transistors TR51 and TR52 when a potential difference occurs between the positive input terminal IN51 and the negative input terminal IN52 is accumulated because of the heat capacities of the transistors TR51 and TR52. The accumulated temperature difference gradually lessens according to the condition under which P(51) and P(52) in Expression (1) become the same. As the temperature difference lessens, the offset error voltage occurring at the output terminal O51 also lessens. When the temperature difference between the transistors TR51 and TR52 is eliminated, the offset error occurring at the output terminal O51 is also eliminated.
As described above, in the conventional DC amplifier 5, the temperature stability is degraded because of temperature variation of the components of the DC amplifier 5 caused by the input signal and an offset error occurs.
Next, occurrence of an offset error because of power supply voltage variation caused by an input signal in the DC amplifier 5 will be discussed. First, power supply voltage variation caused by an input signal will be discussed. In FIG. 5, if the voltage of the output terminal O51 of the DC amplifier 5 is positive, the collector current of the transistor TR53 grows as compared with the case where the voltage of the output terminal O51 is 0. Thus, the power supply current of the positive power supply input terminal V51 and the negative power supply input terminal V52 grows; the current flowing out through the output terminal O51 flows out from the positive power supply input terminal V51 via the transistor TR53 to the output terminal O51 and the current flowing in through the output terminal O51 flows into the negative power supply input terminal V52 via the resistor R55 from the output terminal O51. Therefore, output of the DC amplifier 5 changes with the input voltage, thus the power supply current varies with the input voltage and if the power supply current varies, variation of the power supply voltage commensurate with output resistance of the power supply connected to the DC amplifier 5 occurs. The power supply voltage variation caused by the input signal thus occurs.
Next, occurrence of an offset error because of power supply voltage variation in the DC amplifier 5 will be discussed. In FIG. 5, the applied voltage of the positive input terminal IN51 is set to 0 and the differential amplifier 55 consisting of the transistors TR51 and TR52 and the resistors R51, R52, and R53 is placed in an equilibrium state. The transistor TR53 operates so as to adjust the collector current of the transistor TR53 so that the voltage of the resistor R51 becomes equal to the base-emitter voltage of the transistor TR53 plus the voltage of the resistor R54. If the voltage generated at the resistor R55 by the collector current of the transistor TR53 is equal to the voltage of the negative power supply input terminal V52, the voltage of the output terminal O51 becomes 0.
Next, if the negative power supply input terminal V52 varies and the voltage thereof is reduced to half, the voltage of the resistor R53 is reduced to about half because the positive input terminal IN51 and the negative input terminal IN52 are at ground potential. When the voltage of the resistor R53 is reduced to about half, the current of the resistor R53 is reduced to half and the current flowing into the resistors R51 and R52 is reduced to half. The voltage of the resistor R51 is reduced to half and the transistor TR53 operates so as to adjust the collector current of the transistor TR53 so that the voltage of the resistor R51 becomes equal to the base-emitter voltage of the transistor TR53 plus the voltage of the resistor R54. Thus, the base-emitter voltage of the transistor TR53 plus the voltage of the resistor R54 is reduced to half.
Since the base-emitter voltage of the transistor TR53 is not proportional to the emitter current of the transistor TR53 and is almost constant, if the base-emitter voltage of the transistor TR53 plus the voltage of the resistor R54 is reduced to half, the voltage of the resistor R54 is lessened to half or less. This means that the current flowing between the collector and emitter of the transistor TR53 is lessened to half or less and the voltage of the resistor R55 is lessened to half or less. When the voltage of the resistor R55 is lessened to half or less, the voltage of the output terminal O51 becomes the voltage difference between the resistor R55 and the negative power supply input terminal V52, thus a negative offset error occurs.
Thus, variation of the power supply current of the DC amplifier 5 is caused by the input signal and as the power supply current varies, the power supply voltage varies. When the power supply voltage varies, an offset error occurs in the DC amplifier 5. Therefore, in the DC amplifier 5, an offset error occurs because of the power supply voltage variation caused by the input signal. Hitherto, to suppress power supply voltage variation caused by power supply current variation, a capacitor has been added to the power supply connected to the DC amplifier 5; the power supply current variation is replaced with charge/discharge of the capacitor of the power supply and the power supply voltage is proportional to the charge amount accumulated in the capacitor and thus proportional to the change amount integral value of the power supply current. Therefore, the power supply voltage variation caused by the input signal is proportional to the integral value of the charge amount accumulated in the capacitor, thus the offset error occurring because of the power supply voltage variation is proportional to the integral value of the input signal strength.
Thus, in the conventional DC amplifier, an offset error occurs depending on the temperature stability or power supply voltage variation rate resulting from an input signal. The offset error is proportional to the integral value of the input signal strength.